The invention concerns a calender for treating a material web, particularly a paper web, of the type including a roll gap formed by at least two rolls for passage of the web, and including a heating system and a removable cover system.
Reference is made to the following art, which art is described in greater detail hereinafter:
(1) DE 35 45 123 A1 PA1 (2) DE 37 20 132 A1 PA1 (3) "Das Papier" 1984, p. V 203 ff.
The invention relates primarily to calenders for the paper industry, where calenders serve to glaze the paper. The paper passes the individual roll gaps and undergoes heavy pressing. Contact forces between two adjacent rolls are particularly important for glazing of the paper. The moisture of the paper and its temperature are also of importance. Papers of greater moisture are easier to glaze than dry papers; the glaze increases thus with increasing moisture. The effect is approximately proportional. The same is true for the contact force, which also is called line force. The higher the line force, the greater is also the glaze.
The paper temperature is a very important parameter for the glaze. Here too, the same applies as for the two other parameters: the glaze increases in proportion to the paper temperature.
It should be noted, though, that the temperature of the paper web itself is particularly important, but not the temperature of the roll. When heating the paper web by first heating the rolls, the heat from the rolls must be transferred to the paper web before it becomes usable for glazing. A problem arises here because the rolls transfer their heat not only to the paper web but also to the surroundings, which is highly undesirable. The result is that the required heat capacity--at the same paper moisture and same line force--increases overproportionally. These correlations have been described in reference (3) above.
Known from reference (1) is a calender where hoods are provided which partly surround the rolls during the operation. This is to avoid heat losses of the stated kind. While such hoods are removable, for instance by moving or swinging them out of the way, they are nonetheless very impractical because they impede the accessibility of the calender in the event of operational failure. If for any reason they are briefly opened slightly during the operation, for example due to the vibrations associated with the operation, this leads to grave changes within the paper web and, thus, to a disuniform glaze.
Reference (2) illustrates and describes various types of roll heating. For example calender rolls may be provided with internal heating, for example by thermal oil. But calender rolls may also be provided with external heating, for instance induction heating. In this case, electromagnetic energy is transferred to the material of the roll body, thereby increasing the temperature of the roll surface.
The problem underlying the invention is to so design a calender for treating a material web wherein the cover can be quickly installed and removed again, that no fluctuations in glaze occur during the operation, and that the manufacturing costs will be kept low.